Synthesis of metal-oxygen based materials with controlled porosity by oxidative dealloying

ABSTRACT

Functional materials and methods for making the functional materials are provided. Also provided are methods for utilizing the functional materials in a variety of applications, including catalysis, adsorption, energy storage, and biomedical applications. The functional materials are made from metal alloys via an oxidative dealloying process that selectively removes one or more elements from the metal alloy and converts one or more of the remaining elements into a stable metal-oxygen matrix having a controlled porosity.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under N00014-10-1-0913and N00014-14-1-0675 awarded by the Office of Naval Research. Thegovernment has certain rights in the invention.

BACKGROUND

Hierarchically porous materials that bridge nano- and macroscopic lengthscales find use in a wide variety of applications, including catalysis,energy conversion and storage, and membrane filtration, and in emergingtechnologies for health. Porous zeolites have made the largestcontribution to society so far, and that field is still developing.Other porous solids have also entered the scene in the past two decades,such as metal-organic frameworks (MOFs), covalent-organic frameworks(COFs), and porous organic polymers. No single class of porous materialis ideal for all purposes. For example, crystallinity and long-rangeorder might enhance selectivity for a molecular separation, whilereducing mechanical stability or processability with respect to lessordered structures. Well-ordered nanoporous materials derived fromsmall-molecule surfactant and block copolymer (BCP) self-assembly havebeen explored in the form of amorphous, polycrystalline, andsingle-crystal solids. Unfortunately, multiple time-consuming processingsteps are typically required to generate the final structures. Forexample, removal of organic components by conventional thermalprocessing to create porosity typically takes several hours. Inaddition, nanoporous materials have been fabricated throughelectrochemical dealloying, powder metallurgy, and bottom-up growthtechniques like chemical vapor deposition. To have an impact on realapplications, porous materials must be scalable and must satisfymultiple functional criteria, such as long-term stability, selectivity,adsorption kinetics, and processability, all within a viable costenvelope.

SUMMARY

Methods of making functional materials and the functional materials madeusing the methods are provided. The materials are made from metal alloysvia an oxidative dealloying process that selectively removes one or moreelements from the metal alloy and converts one or more of the remainingelements into a stable matrix having a controlled porosity. Oncefabricated, the porous matrices are post-treated to render them suitablefor various downstream applications.

One embodiment of a method of forming a functional material comprisesoxidatively dealloying a metal alloy comprising: a first element that isa transition metal, a post-transition metal, or a metalloid; and asecond element that is a transition metal, a post-transition metal, ametalloid, or a non-metal. The oxidative dealloying is carried out byexposing the metal alloy to an oxidizing environment comprising oxygenand/or another oxidizing agent and, optionally, water, wherein theoxygen and/or other oxidizing agent reacts with the first element toform a metal-oxide compound of the first element that is stable in theoxidizing environment and reacts with the second element to form anunstable oxide. The unstable oxide evaporates out of the alloy or reactswith water in the oxidizing environment to form volatile hydroxides oroxy-hydroxides that evaporate out of the alloy, leaving a porous matrixcomprising the metal-oxygen compound of the first element. Then theresulting porous matrix is subjected to at least one post-dealloyingprocessing step that changes its chemical properties, physicalproperties, or both.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings.

FIG. 1 is an SEM image of a SiO₂ porous oxide formed by oxidativelydealloying Mo₃Si.

FIG. 2A is an SEM image of a NiO porous oxide formed by oxidativelydealloying Ni₂V.

FIG. 2B is a higher magnification SEM image of the NiO porous oxide.

FIG. 3A is an SEM image of a ZrO₂ porous oxide formed by oxidativelydealloying V₂Zr.

FIG. 3B is a higher magnification SEM image of the ZrO₂ porous oxide.

FIG. 4A is an SEM image of a ZrO₂ porous oxide formed by oxidativelydealloying Mo(2.5 at. %)Zr.

FIG. 4B is a higher magnification SEM image of the ZrO₂ porous oxide.

FIG. 5A is an SEM image of a ZrO₂ porous oxide formed by oxidativelydealloying Mo₂Zr at 900° C.

FIG. 5B is an SEM image of a ZrO₂ porous oxide formed by oxidativelydealloying Mo₂Zr at 1000° C.

FIG. 5C is an SEM image of a ZrO₂ porous oxide formed by oxidativelydealloying Mo₂Zr at 1100° C.

DETAILED DESCRIPTION

Functional materials and methods for making the functional materials areprovided. Also provided are methods for utilizing the materials in avariety of applications, including catalysis, adsorption, energystorage, and biomedical applications. The materials are made from metalalloys via an oxidative dealloying process that selectively removes oneor more elements from the metal alloy and converts one or more of theremaining elements into a stable porous matrix having a controlledporosity. The methods can be carried out on any alloy that oxidizes toproduce one or more volatile species, such as volatile oxides,hydroxides, or oxy-hydroxides, along with one or more non-volatilemetal-oxygen compounds when exposed to an oxidizing environment.

The metal alloys from which the porous matrices are formed comprise afirst element, a second element, and, optionally, one or more additionalelements. Within the alloys, the first element, second element, and anyadditional elements form one or more intermetallic phases and/or alloysolutions. When the metal alloys are exposed to an oxidizingenvironment, the first element undergoes oxidation (that is—undergoes areaction in which electrons are lost) by an oxidizing agent in theoxidizing environment to form an metal-oxygen compound that is stable inthat environment. In the same oxidizing environment, the second elementreacts with an oxidizing agent to form an unstable oxide, or otherunstable metal-oxygen compound. The unstable compound may be a volatileoxide that evaporates out of the alloy or it may be an oxide that reactswith water in the oxidizing environment to form volatile hydroxides oroxy-hydroxides that evaporate out of the alloy. The evaporation of thevolatile species leaves behind a porous matrix comprising the stablemetal-oxygen compound. This process is referred to as oxidativedealloying. Because it is inexpensive and readily available, oxygen is agood candidate for use as the oxidizing agent. However, other oxidizingagents can be used in combination with, or instead of, oxygen. Otherexamples of oxidizing agents include, but are not limited to, sulfur,chlorine, fluorine, bromine, and ammonium. For example, sulfur could bepresent in the oxidizing environment to form metal oxysulfides uponreaction with the metal alloy.

In some embodiments of the metal alloys, the first and second elementsare independently selected from transition metal elements,post-transition metal elements, and metalloids. Accordingly, as usedherein, the term “metal alloy” refers to an alloy in which at least oneof the alloy elements is transition metal, a post-transition metal, or ametalloid. In some embodiments of the metal alloys, the first element isa transition metal element, a post-transition metal element, or ametalloid and the second element is a non-metal, such as carbon ornitrogen. The additional elements can be elements that react with anoxidizing agent in the oxidizing environment to form metal-oxygencompounds, such as metal oxides, that are stable in the oxidizingenvironment. Alternatively, the additional elements can be elements thatreact with an oxidizing agent to form unstable oxides that react in theoxidizing environment to form volatile species. Thus, the additionalelements may be independently selected from transition metal elements,post-transition metal elements, metalloids, and non-metals. If anadditional element forms a stable metal-oxygen compound, the porousmatrix will comprise mixed metal-oxygen compounds of the first elementand the additional element.

As used herein, the term metal-oxygen compound refers to metaloxides—that is, compounds in which oxygen is bonded to one or moretransition metal elements, post-transition metal elements, and/ormetalloids—and also includes inorganic compounds in which one or moretransition metal elements, post-transition metal elements, and/ormetalloids are bonded to oxygen and also to carbon, nitrogen, and/orsulfur atoms. Thus, metal-oxygen compounds include metal oxycarbides,metal oxynitrides, metal oxysulfides, and metal oxycarbonitrides, whichmay be viewed as solid solutions of a metal oxide and a metal carbide,nitride, sulfide, or carbonitride, respectively.

The composition of the metal alloy used to form the porousoxygen-containing matrices can be chosen based on the relativethermodynamic stabilities of the metal-oxygen compounds formed from thealloy elements. Therefore, a wide variety of metal alloys can be used tomake porous matrices comprising a wide variety of metal-oxygencompounds. By way of illustration only, elements that can form stablemetal-oxygen compounds, including stable metal oxides, in oxidizingenvironments include silicon (Si), germanium (Ge), aluminum (Al),titanium (Ti), zirconium (Zr), hafnium (Hf), nickel (Ni), andcombinations thereof. Examples of elements that can react with oxygenand/or water in an oxidizing environment to form volatile metal- ornon-metal-oxygen compounds, such as volatile oxides, hydroxides and/oroxy-hydroxides, include nitrogen (N), carbon (C), boron (B), tungsten(W), molybdenum (Mo), chromium (Cr), vanadium (V), and combinationsthereof. However, some embodiments of the metal alloys and the porousmatrices do not include silicon.

The first element and any additional elements that form stablemetal-oxygen compounds in the oxidizing environment can from acontinuous porous matrix comprising the metal-oxygen compounds, even ifthey are present in the metal alloy as minority components. In someembodiments of the metal alloys, the first element and any additionalelements that form the stable matrix in the oxidizing environment makeup no greater than 20 atomic percent (at. %) of the alloy. This includesmetal alloys in which these element make up no greater than 10 at. %, nogreater than 5 at. %, and no greater than 3 at. % of the alloy. Forapplications that require porous structures having a high mechanicalstrength, it may be desirable to use metal alloys having a highercontent of the first element and any additional elements that form thestable matrix compounds in the oxidizing environment.

Typically, at least some of the second element will be retained in thefinal porous matrix after the termination of the oxidative dealloying.The retained element may be present, for example, as an oxide, as anoxycarbide, as an oxynitride, as an oxysulfide, as an oxycarbonitride,as an intermetallic compound, and/or in an elemental form. The retainedelement will be a minority component in the porous matrix—making up lessthan 50 at. % of the matrix. More typically, the retained element willmake up a very small, but detectable, component of the matrix. Thus, insome embodiments of the porous matrices, the second element comprisesless than 20 at. %, less than 10 at. %, less than 5 at. %, less than 2at. %, less than 1 at. %, less than 0.1 at. %, or less than 0.01 at. %of the porous matrix.

By way of illustration only, metal alloys that can be used to formporous metal oxides include: (a) Mo₃Si, which can be oxidativelydealloyed to form porous silica via the volatilization of MoO₃(g); (b)Ni₂V, which can be oxidatively dealloyed to form porous NiO via thevolatilization of V₂O₅; (c) V₂Zr, which can be oxidatively dealloyed toform ZrO₂ via the volatilization of V₂O₅; (d) Mo(2.5 at. %)Zr, which canbe oxidatively dealloyed to form ZrO₂ via the volatilization of MoO₃;(e) Mo₂Zr, which can be oxidatively dealloyed to form ZrO₂ via thevolatilization of MoO₃; (f) MoHf, which can be oxidatively dealloyed toform HfO₂ via the volatilization of MoO₃; and Mo₃(AlSi), which can beoxidatively dealloyed to form [Al₂O₃][SiO₂]₂ (mullite) via thevolatilization of MoO₃; (g) ZrB₂, which can be oxidatively dealloyed toform ZrO₂ via the volatilization of BO, BO₂, B₂O₂, and/or B₂O₃; (h)HfB₂, which can be oxidatively dealloyed to form HfO₂ via thevolatilization of BO, BO₂, B₂O₂, and/or B₂O₃; (i) TiC, which can beoxidatively dealloyed to form TiO₂ via the volatilization of CO and/orCO₂; and (j) SiC, which can be oxidatively dealloyed to form SiO₂ viathe volatilization of CO and/or CO₂.

By way of illustration only, metal oxycarbides, metal oxynitrides, andmetal oxysulfides from which the porous matrix can be comprised includetransition metal oxycarbides, oxynitrides, and oxysulfides, such astitanium oxycarbides, titanium oxynitrides, and titanium oxysulfides.

The oxidizing environment used for oxidative dealloying will generallybe a high temperature, oxygen- (and optionally, water-) containingenvironment. The working and optimal conditions, including thetemperature and partial pressures of the gases present in theenvironment, can be selected based on the relative thermodynamicstabilities of the metal-oxygen compounds (e.g., oxides) formed from theelements present in the metal alloy. Thus, for a given alloy, one cantailor the oxidizing environment to produce a porous matrix viaoxidative dealloying based on the thermodynamics of the system. Theenthalpies and entropies of formation for many metal and metalloidoxides, oxycarbides, oxynitrides, oxysulfides, and oxycarbonitrides areknown. Those that are not known can be calculated or determinedexperimentally. By way of illustration only, in some embodiments of themethods of forming porous structures, the oxidative dealloying iscarried out at temperatures in the range from 700° C. to 1500° C. Thisincludes embodiments of the methods in which oxidative dealloying iscarried out at temperatures in the range from 800° C. to 1300° C. andfurther includes embodiments in which oxidative dealloying is carriedout at temperatures in the range from 900° C. to 1100° C. However, sincethe optimal temperature for oxidative dealloying will depend on theparticular metal alloy starting material, temperatures outside of theseranges may be used.

The oxidative dealloying can be carried out in open (ambient) air orunder a controlled environment in which the partial pressures of oxygenand/or water vapor are increased. The metal alloys are exposed to theoxidizing environment for a period of time sufficient to achieve thedesired degree of porosity in the final structure. For example, for someapplications it may be desirable to convert the entire starting metalalloy into a porous matrix having pores running throughout thestructure, while other application may require pores extending onlypartially through the structure, for example, only in the near-surfaceregions of the structure. Thus, the working and optimal duration willdepend, in part, on the thickness and geometry of the starting metalalloy substrate. By way of illustration only, in some embodiments of themethods for making porous matrices, the oxidative dealloying is carriedout for time period in the range from five minutes to two hours.

In the oxidizing environment, water can act as a volatilizationaccelerant to form volatile species (e.g., hydroxides) from the unstablecompounds, such as unstable oxides, produced by the reaction of themetal alloy with an oxidizing agent. However, other volatilizationaccelerants can be included in the oxidizing environment in addition to,or as an alternative to, water vapor. The other volatilizationaccelerants would also function to form volatile species upon reactionwith the unstable compounds.

The pores that are defined in the matrices are tortuous andinterconnected and, because oxidative dealloying works from the surfaceof the metal alloy inward, have a pore diameter gradient, whereby themean diameters of the pores decrease as the distance into the materialincreases. The mean pore diameters will depend on the oxidation agentspresent in the oxidizing environment and the specific reactionconditions used, including the temperature and time of reaction. In someembodiments of the porous matrices, the pores are nanoscale pores,having diameters of less than 1000 nm, including less than about 500 nm(for example, from 10 nm to 500 nm). In other embodiments of the porousmatrices, the pores are mesoscale pores, having diameters of 1000 nm orgreater, including greater than 2 μm (for example, from 2 to 10 μm). Instill other embodiments, the dealloying process produces hierarchicalporous matrices that define pores with mean diameters spanning both ofthese length scales.

By tailoring the temperature, time of exposure, and oxygen pressure inthe oxidizing environment, it is possible to tailor the mean pore sizesand pore size gradients in the porous matrices. As discussed above,oxidative dealloying is driven by diffusion of oxygen through the alloy.The oxygen acts to boil off one or more elements of the alloy throughthe aforementioned oxidation and volatilization reactions, and to formsolid metal-oxygen structures with one or more of the remaining alloyelements. At the alloy surface, porosity evolution is unstable until acertain amount of the volatile species has been removed. To achievenanoporosity, the volatilization should occur at a rapid enough rate toprevent coarsening, which acts to smooth the surface. Coarsening(sintering) is driven by Ostwald ripening at high processingtemperatures. Sintering is desirable in that it promotes mechanicaldurability of the stable products of the oxidation and allows for theporosity size scale to be controlled. Higher levels of sintering willprovide larger pores and too much sintering can lead to a non-porousmaterial. Thus, it is possible to strike a balance betweenvolatilization and sintering, both of which are highlytemperature-dependent, for each material system of interest in order toachieve desired porosity characteristics. Generally, higher temperaturespromote oxidation and volatilization, while slowing the kinetics of thereaction through the oxygen pressure will favor smaller pores.

After the porous matrices have been formed they can be subjected to oneor more post-dealloying processing steps that change one or more oftheir physical and/or chemical properties. This converts the porousmatrices into functional material that may be porous or non-porous,depending upon the nature of the post-dealloying processing step. Forexample, the shape of the material (a physical property) can be changedby granulating the porous matrix to form particles of the porous matrixor by molding the porous matrix into a different shape, as by, forexample, pelletizing the porous matrix. Bonding the porous matrix, or aportion of the porous matrix, to a substrate other than the metal alloysubstrate from which it is formed is another example of apost-dealloying processing step that changes one or more physical and/orchemical properties of the porous matrix. This bonding can be carriedout before or after the porous matrix has been separated from anyremaining metal alloy starting substrate. Alternatively, thepost-dealloying processing step can be the step of applying a solid orliquid coating on at least a portion of a surface of the porous matrix.For example, the porous matrix can be reacted with various chemicalspecies (e.g., atoms, molecules, and/or ions) to chemicallyfunctionalize the surface of the porous matrix. However, a chemicalreaction between the metal-oxygen compounds of the porous matrix and thechemical species is not required. Rather, a chemical species can simplybe deposited on or incorporated into the open pores of the porousmatrix. Examples of chemical species that can be reacted with, coatedon, or integrated into the porous matrices include metals, metal alloys,ions, including metal ions, ceramics, oxides, including metal oxides,semiconductor compounds, organic molecules (e.g., amines, thiols,carboxylic acids, etc.), polymers—organic or inorganic, andbiomolecules. If the chemical species includes a metal, that metal maybe different from any metal present in the porous matrix.

The post-dealloying processing steps can be used to render the resultingfunctional materials suitable for use as substrates in a variety ofapplications, including catalysis, biomedical, remediation, and energystorage applications. This is significant because, while various oxidescales have been formed on metal alloys as a result of heat treatmentsin the past, these scales have generally been viewed as protectivesurface coatings or as unwanted by products of the heat treatments. Thepresent materials, in contrast, are not just surface scales, but can beproduced as stand-alone structures that provide starting materials forthe fabrication of a variety of useful devices.

For example, catalytic devices can be fabricated from the porousmatrices by applying a catalytic material, such as a catalytic metal orcatalytic metal alloy, to the surface of the porous matrices to form acatalytic composite. Examples of porous oxides that can be used assupports for catalytic materials include porous SiO₂, TiO₂, Al₂O₃, ormixtures thereof. The catalytic composite can then be exposed tochemical reactants, wherein the catalytic material catalyzes thereaction of the chemical reactant to form one or more reaction products.The catalytic material can be, for example, an oxidation catalyst, areduction catalyst, and/or a photocatalyst. Examples of catalytic metalsthat can be coated onto the surfaces of the porous matrices include gold(Au), nickel (Ni), platinum (Pt), palladium (Pd), rhenium (Rh),ruthenium (Ru), iridium (Ir), and their alloys. These metals can beapplied to the porous matrices by, for example, vapor deposition.

For biomedical applications, a coating of a biocompatible material canbe formed on the surface of the porous matrices, including porous oxidematrices comprising TiO₂, SiO₂, Al₂O₃, ZrO₂, and mixtures thereof. Forexample, a coating of a material having anti-protein fouling propertiescan be applied to the surfaces of the porous matrices. Such coatingsinclude fluoropolymers, including sulfonated tetrafluoroethylene (e.g.,Nafion). Alternatively, a coating of material that enhances thebiocompatibility of the porous matrices can be formed on the surfaces ofthe porous matrices for medical implant applications. Biomolecules canalso be bound to the surface to render the porous matrices suitable foruse as biosensors, including sensors that detect analyte biomoleculesthrough the specific binding of those analyte biomolecules withsurface-bound biomolecules on the porous matrices. Examples ofbiomolecules that can be coated onto and/or bound to, includingcovalently bound to, the porous matrices include proteins, peptides,nucleic acids, oligonucleotides, amino acids, and biopolymers. Fortissue engineering applications, such as bone growth engineering,biological cells, cell attachment factors, and/or cell growth factorscan be coated onto or incorporated into the porous matrices to providecell-seeded tissue growth scaffolds. The seeded scaffolds can then becultured in an appropriate culture medium to grow biological tissues onthe porous materials.

The high surface areas of the porous matrices also make them well-suitedfor use as electrode materials in energy storage applications, such asbatteries and supercapacitors. For example, the porous matrices can beimpregnated with a liquid electrolyte to provide a battery electrode.For use in lithium ion batteries, lithium can be incorporated into theporous matrices, such as the porous TiO₂, in a post-dealloyingprocessing step.

The porous matrices, including TiO₂ and Al₂O₃, can be used as adsorbentsfor removing unwanted chemical species, such as heavy metal ions andorganic molecules, from liquid or vapor-phase samples. In theseremediation applications, samples are contacted with the porousmatrices, whereby targeted chemical species are adsorbed by the porousmatrices. The porous matrices and the adsorbates can then be removedfrom the sample. Chemical species that can be removed include, lead(Pb), cadmium (Cd), copper (Cu), nickel (Ni), zinc (Zn), alkanes andalkenes.

For remediation and other applications, it may be advantageous togranulate the porous matrices or to mold them into regular shapes. Forexample, the porous matrices may be subjected to a post-dealloyinggranulation in which they are crushed, comminuted, and/or sorted byaverage particle size. Alternatively, the porous matrices can be moldedto form regularly shaped particles (e.g., pellets) or objects from theporous matrices. It is also possible to shape the metal alloy startingmaterial into a desired shape, such that the porous matrix, as formed,has the desired final shape.

The porous matrices can also be bonded to various substrates in apost-dealloying processing step to provide functional substrates forvarious applications. For example, the porous matrices can be bound to afiltration medium for remediation applications, or to an electricallyconductive substrate for biosensor or energy storage applications.

Example

This example illustrates methods of forming porous oxides from variousmetal alloys. In each case, the metal alloy was prepared from pureelements weighed to the appropriate proportions and processed bynon-consumable arc melting in a titanium gettered argon atmosphere.Following repeated melting (typically five times), the weight change wasusually below 1%. The molten alloys were then cast into cylindricalingots. Disc-shaped samples were cut from the cast cylindrical ingotsand cleaned by washing in alcohol. Dealloying of the metal alloys wascarried out by oxidation in laboratory air in an open tube furnace attemperatures between 900-1100° C. for times ranging from 10 minutes toone hour. The particular oxidation temperatures and times for some metalalloys are provided in Table 1. Following oxidation, the samples wereexamined under a scanning electron microscope (SEM).

TABLE 1 Dealloying Conditions Oxidation Oxidation Time Metal AlloyPorous Oxide Temperature (° C.) (min) Mo₃Si SiO₂ 1100 10 Ni₂V NiO 750 10V₂Zr ZrO₂ 650 10 Mo(2.5 at. %)Zr ZrO₂ 1100 10 Mo₂Zr ZrO₂ 900, 1000, 110010 MoHf HfO₂ 900 10

The porous oxide listed in Table 1 is the primary oxide formed viadealloying. Trace amounts of other metal alloy-derived oxides or metalsmay also be present. FIG. 1 is an SEM image of a SiO₂ porous oxideformed by oxidatively dealloying Mo₃Si. FIGS. 2A and 2B are SEM images,at different magnifications, of a NiO porous oxide formed by oxidativelydealloying Ni₂V. FIGS. 3A and 3B are SEM images, at differentmagnifications, of a ZrO₂ porous oxide formed by oxidatively dealloyingV₂Zr. FIGS. 4A and 4B are SEM images, at different magnifications, of aZrO₂ porous oxide formed by oxidatively dealloying Mo(2.5 at. %)Zr.FIGS. 5A, 5B, and 5C are SEM images of a ZrO₂ porous oxide formed byoxidatively dealloying Mo₂Zr at 900° C., 1000° C., and 1100° C.,respectively.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A method of forming a functional material, themethod comprising: oxidatively dealloying a metal alloy comprising: afirst element that is a transition metal, a post-transition metal, or ametalloid; and a second element that is a transition metal, apost-transition metal, a metalloid, or a non-metal, by exposing themetal alloy to an oxidizing environment comprising oxygen and,optionally, water, wherein the oxygen reacts with the first element toform a metal-oxygen compound of the first element that is stable in theoxidizing environment and reacts with the second element to form anunstable oxide of the second element that evaporates out of the metalalloy or that reacts with water in the oxidizing environment to formvolatile hydroxides or oxy-hydroxides that evaporate out of the metalalloy, leaving a porous matrix comprising the metal-oxygen compound ofthe first element; and subjecting the porous matrix to at least onepost-dealloying processing step that changes a chemical property,physical property, or both, of the porous matrix.
 2. The method of claim1, wherein the metal-oxygen compound of the first element comprises ametal oxide of the first element.
 3. The method of claim 1, whereinsubjecting the porous matrix to at least one post-dealloying processingstep that changes the chemical property, physical property, or both, ofthe porous matrix comprises applying a solid or liquid coating on atleast a portion of a surface of the porous matrix.
 4. The method ofclaim 3, wherein applying a solid or liquid coating on at least aportion of the surface of the porous matrix comprises chemicallyfunctionalizing at least a portion of the surface of the porous matrixby reacting the porous matrix with a chemical species to form a coatingof the chemical species covalently bound to the surface of the porousmatrix.
 5. The method of claim 3, wherein the chemical species areorganic molecules.
 6. The method of claim 5, wherein the organicmolecules are biomolecules.
 7. The method of claim 5, wherein theorganic molecules are organic polymer molecules.
 8. The method of claim3, wherein the chemical species comprises an electrolyte.
 9. The methodof claim 3, wherein applying a solid or liquid coating on at least aportion of the surface of the porous matrix comprises applying acatalytic metal or catalytic metal alloy to at least a portion of thesurface of the porous matrix.
 10. The method of claim 9, wherein thecatalytic metal or catalytic metal alloy comprises gold, nickel,platinum, palladium, rhenium, ruthenium, iridium, or an alloy thereof.11. The method of claim 1, wherein subjecting the porous matrix to atleast one post-dealloying processing step that changes the chemicalproperty, physical property, or both of the porous matrix comprisesgranulating the porous matrix, molding the porous matrix, or both. 12.The method of claim 1, wherein a portion of the metal alloy remainsafter the oxidative dealloying and further wherein subjecting the porousmatrix to at least one post-dealloying processing step that changes thechemical property, physical property, or both of the porous matrixcomprises separating at least a portion of the porous matrix from theremaining metal alloy and bonding the porous matrix to anothersubstrate.
 13. The method of claim 12, wherein subjecting the porousmatrix to at least one post-dealloying processing step that changes thechemical property, physical property, or both of the porous matrixfurther comprises applying a solid or liquid coating on at least aportion of a surface of the porous matrix.
 14. The method of claim 1,wherein the entire metal alloy is oxidatively dealloyed, such that themetal alloy is completely converted into the porous matrix.
 15. Themethod of claim 1, wherein the porous matrix further comprises thesecond element in its elemental form, an oxide of the second element, anintermetallic compound of the second element, or a combination thereof.16. The method of claim 5, wherein the first element and the secondelement are transition metal elements.
 17. The method of claim 1,wherein the second element is a transition metal element.
 18. The methodof claim 17, wherein the second element is not molybdenum.
 19. Themethod of claim 17, wherein the first element is a transition metalelement or a post-transition metal element.
 20. The method of claim 19,wherein the first element is a transition metal element.
 21. The methodof claim 1, wherein the second element is vanadium.
 22. The method ofclaim 1, wherein: the metal alloy comprises a third element; the oxygenreacts with the third element to form a metal-oxygen compound of thethird element that is stable in the oxidizing environment; and theporous matrix comprises the metal-oxygen compound of the first elementand the metal-oxygen compound of the third element.